OK, so i’ve adjusted the cost of the bourse resources. I’ve removed the .5 thrones a wain counting for the seat’s upkeep as the holder themselves aren’t having to cover it so they wouldn’t have to increase the price to be competitive. There is no way i can also account for how the “free” wains from national positions affect the overall cost of wains in the field, but if a “paid” seat holder charged much less than what they are doing then they would loose money buying a bourse seat.

Any other suggestions for making this more accurate? (Thanks @Amy btw. Every bit helps )

Ah. I thought you were trying to estimate how the Grendal would value wains (roughly), in which case there would be argument for taking the average amount we spend getting wains in and spreading it across all the wains produced by the empire. Similarly any argument could be made for using only our production costs, as we don’t know how they value wains once produced. It’s hard to estimate their “real” worth as their loss is more an opportunity cost then a financial one.
Another option would be to look at the import costs on the ports that import wains - gives a sense of what foreign markets may be like.
(If you enjoy this sort of thing, come find me as Master of Mint in imperial offices - I’ll happily talk all day)

The price paid to liberate slaves paid in up time is significantly less than 4 crowns each.

I think taking the fine paid for freeing slaves as the value of slaves is a poor way of working it out. Thats price for getting caught stealing/doing property damage at that time for that person (my understanding is fines are frequently tailored to the perceived ability of the criminal to pay set off against the severity of the crime.)

Without going into the exact details as that seems like its for the field I would suggest your looking at 1.5 - 2 crowns per slave.

I am hesitant to use figures gathered in up time, as i want to use the wiki as much as possible to avoid a swat from the FOIP stick . however, i think your absolutely right about the cost being inflated to not only cover the cost of the lost property, but also punitive damages so i will make a note.

That seems vaguely accurate. However I would suspect it’s significantly more.
The things it doesn’t cover were lost trade and damaged production.
The loss in 2000 Slaves sounds like a rather awful effect upon ones means of production. I mean I suspect the Grendel now have their own “Winds of Fortune” options where they can either just raid*, or force lesser Grendel Vassal tribes into the mines but that would result in fighting and further damage.
The other will be impact upon trade - each territory provides taxation, this would be heavily impacted by the structural damage and the political damage or the ports no longer being safe. So they’ve lost money through there that takes time to build back up.
All these are I’d say equivalent to some of the nastier economic territory curses. Ofc this is somewhat below the abstraction layer as the Barbarian economic systems while still the same as the Empire (they need taxation, they need supply for armies etc.) are both more simplistic and more “eyeballed”. So an exact figure is impossible.

The other thing to address, is the elephant in the room which is why the opportunity existed (this is information on the wiki). The Grendel had all their armies and all their fleets anchored off Sarvos, this is why their raids were so successful, they were, as per common wiki knowledge, throwing everything they had into the raids after had destroyed a shed load of their armies.
If they don’t want it to happen again they’ll have to send some (no idea how many) Navy’s and Armies back to ensure their coastline is kept safe.
Hell they might back off completely focusing on maintaining their trade as that’s their bread and butter. Or they could continue raiding in a smaller scale, who knows really.

There’s also another result that isn’t quantifiable. The Salt Lord Suriad is suspected to have fallen in battle. Unless there is a clear successor (and even then that person would be in a severely weakened position) this leaves a power vacuum in its place that could mean a challenge for leadership of the area.
We don’t know how their leaders are elected and this situation may take time to resolve, extending the times taken to rebuild and redevelop the damaged areas.

It’s from Urizen yes (hence why its called the SENTINEL Gate, and was a gift from Urizen to the Empire but honestly we have no more real understanding of how it works IC than anyone else, it dates back to the Spirebuilders and other pre-Urizeni people.